Adenoviruses are currently used for delivering genes to cells, both in vitro and in vivo. There are several other methods for introducing foreign DNA into cells including the calcium phosphate precipitation and liposome fusion. Although widely applied to a variety of cell types with high efficiency, these techniques will, typically, deliver DNA to only 1-10% of neonatal cultured cardiac cells, whereas, the adult cells are completely resistant. In contrast, both neonatal and adult cardiac cells are highly sensitive to infection with adenoviruses. Indeed, Dr. Schneider's laboratory has shown approximately 100% efficiency of adenoviral mediated gene delivery to the latter cell type (7). Even though the neonatal cells are amenable to transfection by alternative methods, the applications for that are limited, considering the relatively low efficiency, as the effect of the transfected DNA is most readily evaluated only using a co-transfected reporter gene. In other words, it is difficult to test the effect of the delivered gene on any of the endogenous functions of the cell. Therefore, adenoviral technology is not only advantageous for gene delivery to the transfection-resistant adult cardiac cells but also to the neonatal. In addition, even in the case where a reporter gene is used, the number of cells necessary for the experiment will be considerably lessened by the usage of adenoviruses. Recently, we have successfully used adenoviral vectors to elucidate mechanisms involved in cardiac growth (1) and proliferation (6, 8). Other laboratories have also applied this methodology for better understanding of cardiac cell function. First generation adenoviruses, still currently in use for in vitro studies, are modified by a deletion in the early region 1 (E1) that renders them replication deficient, unless the fragment is provided in trans (4). Foreign DNA up to approximately 4.5 kb in length can be successfully integrated into the viral genome at the site of the deletion. The E1 region mainly controls expression of the late viral genes, but, in spite of its deletion, leaky expression from some of these genes has been detected. In consequence, there have been several attempts to further modify the adeno-vectors in that respect. Deletions of E2, or E4 regions, reduced but did not eliminate viral expression. In the latest generation of the adenoviral vectors, this problem has been eradicated by deletion of the virus's entire coding sequences (9, 10). This new vector, termed "gutless", is not only of benefit for reducing the immunogenicity of adenoviruses in vivo, but also allows for an inset capacity up to approximately 34 kb. Although leaky expression of the viral genes may also be confounding in tissue culture experiments, the use of well- matched control viruses largely alleviate this problem. Accordingly, the adenovirus core facility will continue to generate the first generation adenoviral vector, in addition to the development of the "gutless" vector, for which reagents have been kindly provided by Drs. M. Morsy and C.T. Caskey (Merck).